Responding to a Legionella Outbreak

by Adeline Johnston

When you suspect a legionella outbreak has occurred – or you’ve tested and had a positive result- rapid action can be the difference between containing the incident or having a full scale health emergency. In this article, we cover a step by step plan that facilities should follow to keep their staff and customers safe and help them return their systems to normal.

Recognize the Event as it Occurs

Outbreaks usually occur in one of the following ways

Positive test result: if you’re routinely sampling detects legionella above action levels.
Reported illness: staff or health and safety officials notify of pneumonia-like cases that trace back to your facility

If any sign of contamination shows, it demands immediate attention. The earlier it’s recognized the faster you stop the spread.

Investigate the Source Of Your Legionella Outbreak

Review your monitoring and maintenance logs. You’re looking for:

Any lapse in temperature control or any disinfectant residuals
Any sections with low flow, dead logs, or recent construction
Any high risk assets like a cooling tower, hot water storage, humidifiers, or decorative fountains

If the review is inconclusive, you need to perform follow up testing targeting these areas to isolate the affected system and confirm the contamination pathway.

Choose a Remediation Strategy

Once you’ve identified the source, implement one (or a combination) of the following:

Verify Success Post-Treatment
Re-sample and test to confirm Legionella counts are back within control limits.
Check operational parameters (temperature, disinfectant residuals, pH).
Update your Water Management Plan (WMP) with lessons learned—adjust control limits, monitoring frequency, or maintenance schedules as needed.

Document & Communicate

• Record every action: dates, chlorine levels or temperature achieved, follow-up test results, and any system modifications.
• Notify local health departments or regulatory bodies if required.
• Provide clear, transparent updates to occupants or patients to maintain trust.

Takeaways & Next Steps For Your Legionella Outbreak

A swift, structured response protects health and minimizes downtime:
Recognizing contamination early
Investigating the source thoroughly
Applying decisive remediation
Validating results and updating the WMP

Preparation is the best protection. Ensure monitoring programs are current, staff know their roles, and emergency response protocols are in place before you need them.

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About The Author

Adeline Johnston is the Marketing and Communications Specialist at ChemREADY where she leads brand strategy, content development, and customer engagement efforts. With a background in digital marketing and a focus on education, Adeline creates targeted campaigns and multimedia resources that support ChemREADY’s full range of services, including Legionella management, ANSI/AAMI ST108 compliance, boiler and cooling tower treatment, wastewater processing, and industrial water quality solutions. Passionate about making complex topics accessible, she plays a key role in helping clients stay informed, compliant, and confident in their water treatment programs.